1. Field of the Invention
The present invention relates to a Liquid Crystal Display (LCD), and more particularly to an LCD capable of removing noise generated when driven in an AC mode, and its driving method.
2. Discussion of Related Art
There has been an increasing demand for a display device for displaying an image together with the growth of an information-oriented society, and various flat panel displays such as Liquid Crystal Displays (LCDs), Plasma Display Panels (PDPs), Organic Light Emitting Diodes (OLEDs), and Vacuum Fluorescent Displays (VFDs) have been widely used in recent years.
Among the flat panel displays, LCDs have been widely used in recent years since they are generally driven in an active matrix mode using Thin Film Transistor (TFT) elements, etc. and they are also small-sized, light weight and thin and driven at a low power. An LCD includes a liquid crystal interposed between two substrates and two opposing substrates, and an image is displayed by changing an arrangement of a liquid crystal with an electric field generated between a pixel electrode and a common electrode formed in the two substrates.
That is to say, as shown in FIG. 1, an LCD includes a liquid crystal panel 51 in which a liquid crystal 54 is interposed between a first substrate 52 and a second substrate 53, the first substrate 52 having a TFT and a pixel arranged to face each other and the second substrate 53 having a color filter and a common electrode formed therein. The liquid crystal panel 51 has a liquid crystal cell (pixel) divided by scan signal lines and data signal lines arranged in a matrix shape, and an image is displayed on the liquid crystal panel 51 by controlling a molecular arrangement direction of the liquid crystal compound in every liquid crystal cell.
The molecular arrangement direction of the liquid crystal compound in the liquid crystal cell is controlled by opposite electrodes formed in a surface of the second substrate 53, namely by a voltage supplied to the common electrode and a voltage supplied to a pixel electrode of the first substrate 52 in an ON/OFF operation of the TFT formed in every liquid crystal cell.
Also, in order to ensure reliability of liquid crystal materials, namely to prevent deterioration of the liquid crystal, the liquid crystal display is generally driven in an AC driving mode for reversing a polarity of a voltage supplied to liquid crystal of pixels in every predetermined period.
In the driving system of the LCD in an AC driving mode, there are a line inversion system, a source inversion system, a dot inversion system, etc. In the line inversion system among them, the polarity is reversed in every LOW line on a panel, to thereby supply an image signal to each of the liquid crystal cells.
That is to say, as shown in FIG. 2, for example, a polarity of a voltage supplied to the liquid crystal cells is reversed by changing a voltage (a solid line in the drawing) supplied to the common electrode in every first horizontal period (1H) and a voltage of the image signal (a dotted line in the drawing) supplied to the liquid crystal cells in the line inversion system.
If the liquid crystal is driven in an AC driving mode as described above, then signals with reverse phases are respectively supplied to a pair of electrodes (a common electrode and a pixel electrode), and then a voltage (bias) is supplied between these electrodes.
Accordingly, the liquid crystal display is driven by the line inversion system, and therefore, the second substrate 53, in which the common electrode is formed, depending on the voltage supplied to the common electrode can vibrate.
The second substrate 53 vibrates in a range of about 10 kHz in driving the liquid crystal display since a driving frequency (a frequency of the voltage supplied to the common electrode) of the common electrode is about 10 kHz in a liquid crystal panel for recent portable devices.
The vibration is recognized by users as audio noise, since it is a vibration having a frequency within a human audible bandwidth range.
Such a noise has become a serious problem in recent years since the reduced thickness of portable devices using an LCD results in a smaller distance between a liquid crystal panel and the portable devices.